Antennas for use in telecommunications operate at many different frequencies. Transmit and receive wavebands may be separated so that interference between the signals is reduced, as in GSM and other systems. Intermodulation products may, however, still result, and transmit and receive signals may interfere between themselves. Intermodulation products in receive band signals are particularly undesirable; the operating capacity is reduced and/or the callers cannot clearly communicate, whilst operators face lost calls and accordingly a reduction in revenue.
One form of layered antenna (an antenna having ground planes, feed networks and dielectric spacers arranged in layers) is known from British Patent GB-B-2261554 (Northern Telecom) and comprises a radiating element including a pair of closely spaced correspondingly apertured ground planes with an interposed printed film circuit, electrically isolated from the ground planes, the film circuit providing excitation elements or probes within the areas of the apertures, to form dipoles, and a feed network for the dipoles. Typically, there is a linear arrangement of a plurality of such aperture/element configurations are spaced at regular intervals co-linearly in the overall layered/triplate structure to form a linear array. This type of antenna lends itself to a cheap yet effective construction for a linear array antenna such as may be utilised for a cellular telephone base station, with the antenna arrays being mounted on a frame.
One of the problems which arises during operation is that spurious signals are emitted from mounting apertures and other surface features associated with the reflector plane, for instance, mounting bolts which couple some of the radiated energy, and coaxial cable connector ports. Further, the coaxial cable and/or the cable termination assembly may also radiate spurious signals. The effect of all these unwanted signals is that they will couple with other radiating elements to form intermodulation products. In receive mode these intermodulation signals can severely impair the received signal quality, since they will be of a power level comparable to the received signal strength. In a transmit mode the output power will be reduced to a certain extent and these intermodulation products can affect the beamshape in an indeterminable fashion.
Careful design of the dimensions of the apertures and the elements coupled with the design of the electrical characteristics of the feed network for the elements can give a measure of control of coupling, but for some applications this is not effective. In such cases the performance of the antenna has to be adjusted upon installation, which complicates such a procedure and does not, in fact, solve the problem of spurious radiative effects behind the antenna. These problems are not limited to layered (tri-plate) antennas.